Bakgog reservoir with watershed area of 84.8 km2 and Miho reservoir with watershed area of 133.3 km2 are located in Jincheon County, Chungbuk Province within the watershed of Miho stream with its area of 1,856.08 km2.Followed by heightening 2 m of Bakgog reservoir with total water storage capacity increased from 21.75 mm3 to 26.62 mm3 and 1 m of Miho reservoir with total water storage capacity increased from 13.87 mm3 to 16.49 mm3, streamfiows at Miho A station with watershed area of 579.6 km2, at Miho B station with watershed area of 1,625.1 km2, and at Miho C station with watershed area of 1,856.1 km2 downstream were simulated on a daily basis and flow duration curves were drawn and compared between before and after heightening upstream reservoirs.Improvement of flow duration curves could be not visible at any station.But a little some streamflow increase could be visible in low flows from 275th flow to 355th flow at Miho A station.Increases of reservoir water storage capacity were 4.87 mm3 in Bakgog reservoir and 2.62 mm3 in Miho reservoir.Amounts of annual mean streamflows were decreased from 295.2 mm3 to 294.9 mm3 at Miho A station, from 1,050.2 mm3 to 1,049.9 mm3 at Miho B station, and from 1,182.2 mm3 to 1,180.8 mm3 at Miho C station.These decreases of annual streamflows were considered as the storage effect of reservoirs.From the above study it was concluded that any effect of increasing downstream streamflows by heightening upstream irrigation reservoirs would be very negligible low.But the possibility of improving low flows in a short reach downsream of irrigation reservoirs would be remained.